Case 2: x has + or 0 balance factor

If x has a + or 0 balance factor, we rotate left at y and update
parent pointers as for any left rotation (see PBST Rotations). We
also update balance factors. If x started with balance factor 0,
then we're done. Otherwise, x becomes the new y for the next loop
iteration, and rebalancing continues. See avldel2, for details on
this rebalancing case.